We have accounts with various wholesalers. Ingram Micro and Tech Data are the big ones. After that we also have reseller deals with some big OEMs.
For Ingram Micro or Tech Data (or any of the other big 5,) you get the account then you apply as a reseller with the various companies that use them. After being accepted you can then purchase through Ingram Micro. Companies like Apple, HP, Acer, and all the others sell through the big wholesalers. So if I wanted to sell HP servers, or Cisco routers I would get a reseller agreement in place (become a partner) with one of them, then tell Ingram Micro, who will verify it with the company. After that I can order those parts (they show up in searches and allow me to add them to my cart.)
Also with Ingram Micro you get generic access to a lot of the major companies. I didnt have to become a reseller to buy any of the hard drives or various smaller parts.